It is well known to form containers or cans, and particularly beer or beverage cans, by a drawing and ironing process wherein flat sheet metal stock material is fed from a coil to a press and circular blanks are die-cut from the stock material and formed into cups. The shallow cups have side and bottom walls that are substantially equal in thickness and these cups are then transferred to a body-making apparatus where the cups may be redrawn to further reduce their diameter and increase the height to produce a finished container shell.
Usually, the bodymaker consists of a punch or mandrel that receives the shallow cup and has an external diameter substantially equal to the final internal diameter of the container shell. The mandrel or punch is moved along a path and initially forces the cups through a redrawn ring and then through a plurality of ironing dies or rings which have inside diameters that are consecutively smaller than the outside diameter of the redrawn cup passing therethrough so that pressure between the ironing dies and the punch progressively reduces the thickness of the side wall and forces the metal along the punch to increase the overall height of the container body. After the side wall of the container has been reduced to a minimum thickness, the end wall, which remains substantially equal to the original thickness of the sheet metal stock, is reformed generally into an overall dome-shaped configuration.
In most commercial presses, the bodymaker or drawing and ironing machine includes a domer assembly that is positioned at the end of the stroke of the punch or ram to cooperate with the punch and reform the end wall at the end of the drawing and ironing operation.
One type of machine that has been utilized for producing container shells of the above type is manufactured by Ragsdale, Inc., and is identified as a Model CR-24 Can Wall Drawing and Ironing Press. This machine includes a plurality of axially-spaced die assemblies that cooperate with the movable punch to convert the shell cup into a finished container shell. At the end of the stroke, the punch cooperates with the domer assembly for producing the final configuration of the integral end wall of the container. Usually such end wall configuration is domed inwardly, as for example is shown in U.S. Pat. No. 3,942,673, which is assigned to the Assignee of the present invention. This dome configuration for the end wall has a very critical configuration which allows the container to withstand substantial internal pressures during the filling and handling operations, and also to withstand considerable stresses or resistance to buckling and inversion when the container is dropped after being filled.
Another type of machine that has found a remarkable degree of commercial success for producing container shells of the above type is manufactured by Standun, Inc. and is generally disclosed in U.S. Pat. No. 3,735,629. The Standun machinery, in its present commercial form, as modified by the Assignee of the present invention, has been proven to be capable of producing container shells at a rate of more than 200 per minute under strenuous operating conditions and, thus, the time for reforming the end wall to its final complicated configuration is extremely limited and the container end wall must be free of any wrinkles after such reforming process.
In the commercial machinery discussed above, the redraw and ironing rings, as well as the stripper assembly, are usually formed into a package, such as disclosed in U.S. Pat. No. 4,300,375, that fits into a very confined space. Likewise, the domer assembly must fit into a confined space to conserve space for the overall unit.
To produce the final dome configuration of the end wall, various types of doming assemblies have been proposed for cooperating with a punch to reform an end wall of a container shell.
One example of such doming assembly is disclosed in U.S. Pat. No. 3,491,574 wherein a fixed doming element is positioned in the path of movement of the punch adjacent the end of its stroke and has a surrounding stripping element for removing the finished shell from the punch after the doming operation has been completed. One of the problems of having a fixed domer element for reforming the end wall of a container shell is that difficulties may be encountered when a cup is initially misfed into the punch, which can cause serious damage to the domer assembly, as well as to the remaining elements of the press or machine. Furthermore, wrinkling may occur during the reforming of the end wall if the pressures are produced unevenly between the domer assembly and the punch.
In an effort to overcome some of these problems, U.S. Pat. No. 3,771,345 discloses a domer assembly in which the domer pad or element is capable of retracting somewhat against a pneumatic pressure source to absorb the shock at the end of the stroke of the punch. Also, the more complicated configurations for the domed end wall, particularly end walls having a peripheral annular portion surrounding the reduced central domed portion of a container, requires separate elements in the domer assembly for producing the final end wall configuration. U.S. Pat. No. 3,771,345 discloses a domer assembly, which is commonly referred to as a double action domer, wherein an annular forming element surrounds the central domer pad to produce the inwardly-directed configuration of the lower peripheral end of the side wall of the container and then an upwardly reversed dome interconnected to the annular portion by a generally vertical wall.
In an effort to reduce the cost of finished containers, manufacturers are constantly striving to reduce the thickness of the initial stock material, thereby decreasing the overall metal cost of each container. Since the end wall (bottom) of the container essentially represents the initial thickness of the can stock, new bottom profiles are required to maintain the pressure performance of thinner guage of stock materials. Purchasers and users of such containers, particularly the beer and beverage industry, have very stringent requirements which require that the finished and filled container be capable of maintaining internal pressures above 100 psi minimum without any significant distortion, and to provide such capability, rather elaborate dome or end configurations have been developed. Another more recent requirement for such drawn and ironed containers is that the container must have a minimum resistance to the reversal of the end wall when the container is inadvertently dropped after it has been filled and is ready for sale and that the end wall have a buckle resistance of more than 100 psi.
Thus, one of the more recent proposals for the end wall configuration of the container consists of forming an arcuate annular portion depending from the end wall to a small flattened portion which defines the lower end of the container and then into an upwardly-directed annular portion which ends in a central upwardly-directed dome. This type of end wall configuration is disclosed in the above U.S. Ser. No. 426,888, and the container incorporating such end has received a remarkable degree of commercial acceptance.
However, as the thickness of the stock material decreases, the problem of wrinkling the container shell wall becomes more acute, particularly when attempting to produce the more elaborate configurations in the end wall.
In order to produce the more elaborate configuration of end wall of container shells with thinner metal, such as discussed above, more elaborate equipment is necessary to prevent wrinkling of the metal during the reforming process. Thus, the above-mentioned U.S. Pat. No. 3,771,345, which discloses a center domer element surrounded by an annular element, has both elements maintained in a predetermined position through a pair of piston and cylinder arrangements wherein air is introduced to apply the desired forces to the container shell in cooperation with the punch to reform the end wall. Another example of a domer assembly which has previously been used is disclosed in U.S. Pat. No. 3,730,383, wherein a fixed domer element is surrounded by a movable annular element that is biased through an air cylinder, and U.S. Pat. No. 4,289,014 where air bags are used for shock absorption for both the center domer pad and the annular forming element.
In addition to the above, other types of biasing mechanisms, such as spring-biasing mechanisms, have been proposed for a center domer element and the annular domer element, such as disclosed in U.S. Pat. No. 3,967,482. However, the forces applied in utilizing the spring assembly concept do not lend itself to uniform and equal application of forces around the entire perimeter of the container during the reformation of the end wall, which may result in wrinkles, making the container unuseable.
Thus, while numerous types of domer assemblies have been proposed and have been utilized on a commercial basis, manufacturers of drawn and ironed containers are constantly striving the enhance the machinery for forming such drawn and ironed containers so that it will be more reliable, can easily be maintained and can also be changed to change the configurations of the container end walls, if desired.